Since its discovery in 2004, graphene has attracted the attention of engineers and scientists across many research disciplines and application areas. Rapid development has been made not only in understanding the physics, chemistry and other fundamental properties of graphene, but also in development of graphene-based devices such as transistors, solar cells, gas sensors and supercapacitors. Doping graphene to change the carrier density has been found to be one method to control electronic properties, and various inventive embodiments expressed herein pertain to a a facile means of nitrogen doping.
Graphene has emerged as an important material with diverse prospective applications. Widely used graphene synthesis techniques include: mechanical exfoliation, chemical exfoliation, epitaxial growth over SiC, and chemical vapor deposition (CVD). Mechanical exfoliation was the first reported technique and gives high-quality films but is difficult to scale up, and both chemical exfoliation and SiC growth are multistep processes. The CVD technique is a single-step process and offers promise for large-scale graphene growth and meeting the projected demand for graphene production. Cu and Ni are the two metallic substrates used for graphene synthesis using thermal CVD. By controlling the CH4 flow rate, predominantly single layer films on Cu over large areas can be obtained.
Large-area synthesis has made relatively inexpensive Cu one of the most attractive substrates for graphene growth. Several studies have provided insights into the mechanism underlying thermal CVD of graphene on Cu, where substrates are typically preheated to approximately 900 to 1000° C. before introducing hydrocarbon gas mixtures. As the demand for graphene increases, a CVD technique for rapid graphene growth at reduced substrate temperatures would be useful. Microwave plasma-enhanced CVD (MPCVD) is one such technique that has proven useful for large-area and low-temperature growth of various carbon based nanostructures, including CNTs, nanocrystalline diamond films, and carbon nanowalls (or vertically standing few-layer graphene sheets).
However, in order to make doped graphene more scalable, a synthesis method capable of rapid, large-area processing is very much needed. Chemical methods and production of graphene by arc discharge, though producing large quantities of both doped and undoped graphene, may be limited by the size of the graphene flakes. Chemical methods also lead to functionalization of the graphene flakes. Of the different synthesis techniques, chemical vapor deposition has shown promise for large scale synthesis of doped large-area graphene films. However, the technique may use high temperatures and may need several hours for the process to be completed.
Microwave plasma CVD (MPCVD) is another promising technique that has been widely used for low-temperature and fast growth of different carbon based nanostructures including flat graphene films and graphene flakes. The coupling between methane/hydrogen plasma and a metal foil in the MPCVD process enabled a very rapid and localized heating of the metal foil to produce graphene growth within a few minutes without any supplemental heating. Because of this localized heating on a thermally light substrate (i.e., an elevated foil), the cooling process was also shown to be extremely fast.
What is needed are improved methods and apparatus for growing graphene. Various embodiments of the present invention achieved this in novel and nonobvious ways.